The present invention relates to a modem pooling system for pooling a modem so that the modem is usable in common for a plurality of data terminals, where the modem modulates and demodulates communication data to enable communications between the data terminals and a public subscriber telephone network (PSTN).
In a key telephone system or a PBX (private branch-exchange), a modem for modulating and demodulating data is provided to enable communications between data terminals connected to extension telephone sets-and the PSTN. In this case, since it is not economical to provide a relatively high-costly modem for each data terminal, a single or several modems are pooled, and the pooled modem is selectively connected to the data terminal only when the data terminal is required to communicate with the PSTN. In this specification, the above-mentioned system is referred to as a modem pooling system. In the system, conventionally, when a calling party number signal is transmitted from the data terminal or a called party number signal from the PSTN is received by the data terminal, a central control unit for controlling the call processing and the exchange operation transmits or receives control commands between the modem and the data terminal via the RS-232D interface, respectively, analyzes transmit/receive data to/from the RS-232D, and controls various call processing such as modem mode control, data terminal alerting, communication channel connection between the modem and the data terminal, etc. on the basis of the analyzed results. In parallel to the above processing, additionally, the communication channel connection between the modem and the PSTN is controlled. In this case, the modem automatically receives a called party number signal and transmits a calling party number signal in accordance with the procedure prescribed by V. 25 bis of CCITT Recommendation. In the above automatic operation, it is also required to finely control the sequence or the timing of the communication channel connection, for prevention of data emission.
In the conventional modem pooling system, since the processing executed by the central control unit at the calling party number signal transmission or the called party number signal reception is complicated, there exists a problem in that a large load is applied to the central control unit.